In the plastic extrusion industry, raw plastic material, called “resin” in the industry, is melted and formed into a continuous profile. Extrusion produces items such as piping, tubing, window frames, rolls of film, bags, sheets and the like. In the extrusion of plastics, raw thermoplastic materials in the form of small pellets of resin are used to make the plastic end-product. The pellets start out in a product supply container, such as a silo or gaylord box. A pneumatic system is used to move the plastic pellets from the supply container to one or more vacuum receivers. Pellets are dispensed from the vacuum receivers into a blender where pellets of different types, i.e. the “ingredients” for the recipe, and in pre-determined quantities, are combined to produce the end product in accordance with the pre-determined product recipe. Additives such as colorants and ultraviolet inhibitors, for example, which are also distributed in pellet form, can be dispensed in like fashion.
Once blended, the ingredients enter the feed throat of a plastic extruder, the feed throat being disposed to one end of the extruder. The ingredients then come into contact with a rotating screw which forces the pellets through a barrel within which the screw is rotating. The barrel is heated to a desired temperature which allows the pellets to melt gradually as they are pushed through the barrel. Extra heat is contributed by the intense pressure and friction that takes place within the barrel. At the opposite end of the extruder barrel, the molten plastic enters a die, which gives the final product its profile. The plastic exits in continuous form to be cut in lengths or coiled, depending upon the end product desired.
In the extrusion molding system, as is also true with alternative methods of compression, transfer and injection molding, the raw material, i.e. the resin pellets, must be moved about and distributed to extruders as is required for processing. This is typically accomplished through the use of pneumatic systems having vacuum pumps that move the pellets through and along vacuum lines and the like.
Prior systems were designed to maintain a pick-up velocity in excess of the worst case requirement for said material under maximum design vacuum level. The disadvantage of such systems is that at lower vacuum levels or lower pick-up velocity requirement the material is conveyed at a velocity which is greater than is necessary. The problems caused by conveying at higher than required velocities are as follows: (1) formation of angel hair, streamers, or stringers, (“streamers”) (2) reduction in system efficiency and (3) material degradation.
Such systems operate far in excess of the saltation velocity to avoid such blockages. The disadvantage of such systems, aside from energy consumption is that the higher velocity may damage the particulates conveyed.
For most such materials, the “pick up” velocity is at least 4,000 feet/minute (fpm). However, there is a wide range of variance between material properties. The inventors know of various materials used in plastic extrusion that have differing properties and can require pickup velocities between, for example, 3,500 fpm and 6,000 fpm. Applicant has partially addressed conveyance of materials with different properties with its Smart Connect Stand. A feature of the Smart Connect Stand is that it can be programmed to identify the type of material being conveyed. As a result, the properties of the material to be conveyed are known, such as the appropriate pickup velocity.
Material pickup velocity is critically important over long conveys. Over long conveys, such as from an exterior storage bin, there is a increasing velocity gradient, that is, the material steadily gains in velocity. As indicated above, above certain velocities, the particulate material used in plastic extrusion may strike the walls of the conduit and elongate, forming long ribbons or strings known in the industry as “streamers.” Streamer formation is extremely undesirable as the resins conveyed in pellet form have certain properties desired for the extrusion. Streamer creation frequently alters the property of the resins thereby reducing the quality of, or ruining the end product extrusion. Additionally, streamer creation can block conduits and cause production downtime.
An additional variable unaccounted for in prior systems is the existence of bleed valves in storage vessels such as railroad cars and other storage devices. These bleed valves are referred to in the industry as manual flow control valves. Opening and closing a bleed valve can cause significant variations in system pressure, and therefore particulate speed and streamer formation. Varying the flow control valve results in an increase or decrease in the material to air ratio. Increasing the material to air ratio is accomplished by closing the manual flow control valve. Increasing the material to air ratio increases the system vacuum level at the pump.
What is needed is a convey system that can maintain a constant pick-up velocity even under varying load conditions (vacuum levels). Such characteristics in pneumatic solids convey improve system efficiency, reduce the propensity for material degradation, and reduce the propensity for the formation of “angel hair”.